This invention relates to a cascade control system for controlling the throttle pressure in a boiler-turbine system where the boiler is of the drum type and the control is to be executed by modifying the rate of fuel feed to the boiler. In the fuel supply systems for many boilers, it is difficult to make a direct measurement of the rate at which the heat is supplied to the boiler and it is therefore necessary to make an inferential measure of the heat input. One such method involves measuring the boiler output as by measuring turbine first stage pressure as an indication of steam flow or by measuring the megawatts from the connected generator. Such inferential measurements are proportional to the heat input only under steady state conditions and therefore the measurement must be modified by the addition of a quantity indicative of the changes in energy storage. Thus, the steam generator output plus the rate of change in stored energy is proportional to energy input under both steady state and dynamic conditions. That calculated value is considered as a measurement of the heat release or, in other words, the heat input to the boiler.
The heat release measurement mentioned above has been utilized in a cascade control system in which the primary controller responds to the deviation of the throttle pressure from its desired value to produce a set point for the secondary controller indicative of the heat release demand. That set point is then compared with the calculated heat release in the boiler as determined, for example, by summing signals indicative of the steam flow and the rate of change of pressure in the drum. The deviation resulting from that comparison provides an input to the secondary controller which is arranged to operate controls for modifying the rate at which fuel is fed to the boiler such as, for example, by controlling the speed at which a coal mill is operated.
A cascade control system of the type described above is useful in controlling the fuel feed to a boiler in circumstances where it is anticipated that there will be a disturbance in the relationship between the setting of the fuel control and the rate at which heat is supplied. Such a change may occur, for example, when the coal supplied by a coal mill is changed from one grade to another when the grades have a different BTU content.
While the cascade control system described above provides a useful means for modifying the fuel feed to the boiler so as to take into account changes in the fuel feed system such as changes in the quality of the coal supply, it has been found that in that cascade control system the control loop which involves the secondary controller is not substantially faster than the control loop involving the primary controller. Instead both loops have about the same response time and as a result there tends to be an interaction between the two control loops. There may, for example, occur in the primary controller a reset action due to changes in the heat input to the boiler in response to changes in the quality of the coal supplied. However, since these changes should be taken care of by the control loop involving the secondary controller, the reset action accumulated by the primary controller will have to be removed by the action of the secondary controller. Hence, some of the control action taken by the primary controller eventually has to be undone by the secondary controller, thus there is an interaction between the primary and secondary controllers.
It is an object of this invention to provide an improvement in the cascade control system described above which will substantially prevent interaction between the primary and secondary controller when control is called for in response to changes in the fuel supply system which should ideally be handled by the secondary controller without assistance from the primary controller in order to avoid interaction between the controllers.